For the purpose of improving visibility or the like of an image display apparatus, a technique of focusing light beams emitted from a light source toward a front direction so as to enhance the brightness has been used in general. More specifically, for example, a lens, a mirror (reflective layer) a prism or the like is used for utilizing refraction and reflection for focusing and parallelizing light beams, thereby enhancing the brightness.
For example, in a liquid crystal display apparatus, a light beam emitted from a light source is focused in a front direction by a prism sheet or the like so as to make the light beam enter a liquid crystal display apparatus efficiently to enhance the brightness. However, since a large refractive index difference is required in principle when focusing by means of a prism sheet, the prism sheet must be disposed via an air layer or the like. This may result in an optical loss caused by unnecessary reflection or scattering. Another problem is that a large number of parts will be required.
For another technique of enhancing emission brightness in polarization, a brightness enhancement system that uses retro-reflection has been proposed. Specifically, this brightness enhancement system includes disposing a reflective layer on a bottom surface of a light-guiding plate and disposing a reflective polarizer on the light-emitting surface. A light beam entering the system is separated into transmitted light and reflected light depending on its polarization state, and the reflected light is reflected via a reflective layer on the bottom surface of the light-guiding plate and re-emitted from the emission surface so as to enhance the brightness. For example, reflection and separation of circularly polarized light by a cholesteric liquid crystal is detailed in, for example, JP 03(1991)-45906 A, JP6(1994)-324333 A, and JP 07(1995)-36032 A. However, such a brightness enhancement system cannot provide sufficient effects with respect to a light source whose focusing property is improved previously by using a prism sheet or the like, in comparison to a case where it is applied to a light source having a strong diffusivity.
For solving the above-described problems, techniques for enhancing brightness have been studied for parallelizing light beams from a light source by using a special optical film instead of a lens, a mirror, a prism or the like. A representative example is a method of using a combination of a line spectrum light source and a bandpass filter. More specific examples includes a method of disposing a bandpass filter on a line-luminescent light source such as a CRT or electroluminescence or a display apparatus, as described in applications or issued patents of Philips, for example, JP 06 (1994)-235900 A, JP 02(1990)-158289 A, Tokuhyo 10(1998)-510671 A (published Japanese translation of PCT international publication for patent application), U.S. Pat. No. 6,307,604, DE3836955, DE4220289, EP578302, US2002-0034009, WO002/25687, or US2001-521643 and US 2001-516066. Another example of a technique as described in US2002-0036735 (Fuji Photo Film Co., Ltd.) includes disposing a bandpass filter corresponding to three wavelengths, with respect to a line spectrum type cold cathode ray tube. However, these techniques have disadvantages that they will not function to the light sources without a line spectrum, or they have problems in designing and manufacturing films that selectively function with respect to a specific wavelength. Furthermore, an evaporated interference film is often used for the bandpass filter, but it has disadvantages, for example, that the wavelength properties may change under a humidified atmosphere, due to a change in the refractive index of the thin films.
Examples of a light-parallelizing system that uses a hologram-based material include a system described in U.S. Pat. No. 4,984,872 A1 (Rockwell International Corporation). However, the material has a high front transmittance while its reflection-elimination rate for oblique incident light beams is not so high. When a parallel light beam is provided to this system for a calculation of the straight transmittance, the transmittance in the front direction will be measured high since the light passes through in the front direction, while an oblique incident light beam will be scattered so that measurement value for the transmittance will be low. However, the difference will not occur on a diffusion light source. Therefore, for a case of disposing the system on a diffusion backlight light source in use, it cannot exhibit necessarily its focusing function sufficiently. Moreover, the hologram-based material has problems in its physical properties such as durability, reliability or the like.